Sensors based on the electrochemical activity of an electro-conductive solid polymer transducer such as polyaniline (PANI) or polypyrrole are getting increased attention in the scientific community owing to their numerous advantages.
Polyaniline is a very convenient material when used as a solid electrochemical transducer owing to its many interesting intrinsic combinations of redox and acido-basic states. This polymer is known as stable and highly conductive in its emeraldine acid form.
Polyaniline also makes it possible to chemically, i.e. by chemical grafting, or physically, i.e. by physisorption, sensitize its surface with natural or engineered biochemical agents for biosensing purposes. As an example, some enzymes are well known to release protons from their specific substrate hydrolysis. This allows to develop sensors based on a proton releasing enzyme mediator.
In this context, the polyaniline protonation state strongly affects the equilibria between its redox states. This material can be electrochemically characterized to indicate the pH of an aqueous solution. Indeed, a simple potentiometry measurement of a polyaniline covered working electrode against a reference electrode displays a simple nernstian behavior as a function of the proton concentration. This simple system, similar to a conventional combined pH electrode, can be viewed as a battery cell having a limited charge and producing its own potential difference proportional to pH.
The two electrodes potentiometry measurements are passive: a constant discharging of the cell, through its internal resistance and through the high impedance measuring voltmeter, occurs. This can, in some cases, cause unwanted alteration of polyaniline redox states and cause a drift in the measured potentials. It is as though the reference and working electrodes were pulling each other's electrochemical potential to annul their difference by a small current exchange through the solution and the measuring instrument internal impedance. This drawback effect can be easily rendered negligible by using large surface working electrodes, thus increasing the available charge, but can however completely prevent reliable measurements at the micro and nano scales. Moreover, polyaniline electrochemical impedance varies simultaneously as a function of both the pH and its electrochemical potential.
For these reasons and for the sake of simplicity, electrochemical sensors, that are supposed to be used more than once or during long periods of time, favor the use of a continuous actuation during sensing rather than a supplementary differed protocol for electrochemical resetting or reloading. In an ideal active sensor, the actuation channel has to control, as best it can, some interface physico-chemical properties such as redox, acid-base and dielectric activities. This actuation is made in order to keep the transducer properties as unchanged as possible while observing at the same time how the system proceeds to do so in its feedback control. Moreover, in a sensor array, different actuations could lead to either increased selectivity or sensitivity or precision, starting from initially identical micro or nano-sensors.
A well-known equipment to investigate reaction mechanisms related to redox chemistry and other chemical phenomenon is the potentiostat. A potentiostat is a control and measuring device using three electrodes: a working electrode, a reference electrode and a counter electrode. A basic use of a potentiostat consists to consider it as an electric circuit which controls the potential across the cell by sensing changes in its impedance, varying accordingly the current supplied to the system: a higher impedance will result in a decreased current, while a lower impedance will result in an increased current, in order to keep the voltage constant. As a result, the variable system impedance and the controlled current are inversely proportional.
      I    o    =            E      c              R      v      
Where Io is the output electrical current of the potentiostat, Ec is the input voltage that is kept constant and Rv is the electrical impedance that varies.
The potentiostat, FIG. 1, in its conventional architecture needs thus an input signal Ec, which is the difference in potential to be applied between the working electrode W and the reference electrode R. Then, the potentiostat generates an output signal Uout that is proportional to the magnitude of the current flowing between the counter electrode C and the working electrode W. The current flow serves to maintain the working electrode W at the desired potential against the reference electrode's one. This system can be considered as the imbrications of operational amplifiers (Integrated Circuit, IC) into an analogue feedback loop. In the schema of FIG. 1, A1 is used as an adder, A2 as a voltage follower and A3 as a current to voltage converter scaled by a selected resistance R3.